OLEDs
Flat panel displays based on organic light emitting devices (OLEDs) have been studied extensively in recent years. OLEDs offer exceptional potential for thinner, more energy efficient displays with equal or better resolution and brightness compared to the current liquid crystal display (LCD) technology. OLEDs also offer high switching speeds, excellent viewing angles (>160°), red, green, and blue (RGB) color selection possibilities, and because no backlighting is necessary, it may be possible to fabricate devices on flexible substrates. However, despite the enormous research and development effort on OLEDs, there are presently few commercially available products using this technology. One of the apparent problems is the need for the development efficient materials to satisfy the electronic device requirements, especially lifetime.
The scientific basis for OLEDs relies on an organic/polymer material's ability to emit light when subjected to electrical stimuli. In this process, electrons and holes are injected into the material from respective electrodes and diffuse through the material. The electrons and holes then recombine creating an excited state within an organic/polymer emissive layer. The excited state can then undergo radiative decay emitting a photon. Depending on the organic/polymer material and its substituents, the wavelength of light emitted can be any color and even multicolored, e.g. red, green, blue, or combinations thereof.
For optimal operation, it is important that the rate at which the holes and electrons diffuse into and through this emitting layer be similar, and preferably matched. Hence, numerous efforts have been made to optimize transport of both holes and electrons to the emitting layer and also to prevent trapping of holes or electrons that leads to destructive effects within the devices. Most recently, efforts have been made to incorporate organic molecules or polymers that promote transport of holes or electrons within the OLED device. Still more recently, efforts have been made to incorporate organic molecules or monomer units within polymeric systems such that one organic unit promotes hole or electron conduction and a second organic unit promotes emission. Such electronic tuning is designed to minimize the transport barriers and maximize the hole/electron injection balance, thereby enhancing the potential for radiative decay rather than non-radiative decay in the form of heat. Considerable work in this area remains. Device stability, electronic efficiency, and manufacturing simplicity have been ongoing challenges in fabricating such devices.
Early examples of organic electroluminescence were reported by Pope et al. in 1963 [Pope, M.; Kallmann, H.; Magnante, P. J. Chem. Phys. 1962, 38, 2042] who demonstrated blue light emission from single crystal anthracene using very high voltages, ≈400 V. 
Advances on OLED processing over the next two decades were limited to forming thin, light emitting films of organic compounds by vacuum deposition techniques, [Vincett, P. S.; Barlow, W. A.; Hann, R. A.; Roberts, G. G. Thin Solid Films 1982, 94, 476] and lowering driving voltages to <30V, however these single-layer devices suffered from both poor lifetimes and luminescence efficiencies. In 1987, Tang and Van Slyke [Tang, C. W.; Van Slyke, S. A. Appl. Phys. Lett. 1987, 51, 913] at Eastman Kodak discovered that strategically designed two-layer electroluminescent devices could be fabricated with voltages, brightness and efficiencies for commercial display applications. As shown in FIG. 1, the OLED device 10 was prepared by sandwiching organic hole transport (HT) material 12 and emissive (EM) material 14 between an indium-tin-oxide (ITO) anode 16 and magnesium/silver alloy cathode 18 layers. A conventional electric potential source 20 was connected to the cathode 18 and anode 16. A glass substrate 22 allowed light emission as shown by Arrows 24. The hole transport (HT) and emissive (EM) materials used by Tang and Van Slyke are shown below. 
The key to device performance was the layered architecture sequence: cathode/emissive-electron transport-hole transport/anode. These devices demonstrated brightness, efficiencies, and lifetimes far exceeding anything reported at that time. The materials shown in FIG. 1 were deposited onto indium tin oxide (ITO) coated glass by a vacuum sublimation process to a thickness of ≈25 nm.
In 1990 Burroughs et al. [Burroughs, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.; Friend, R. H.; Burn, P. L. Nature 1990, 347, 539] developed polymeric OLED devices or PLEDs. In 1992, Braun et al. [Braun, D.; Gutafson, D.; McBranch, D.; Heeger, A., J. J. Appl. Phys., 1992, 72, 546] discovered that poly(p-phenylenevinylene) (PPV), and its derivatives will electroluminesce both green and red light when confined between ITO and aluminum electrodes. 
This work was important because PPV polymers can be deposited by a spin coating process that is more cost effective than vacuum sublimation. Spin coating also facilitates coating larger areas. As a result of these pioneering examples, hundreds of OLED and PLED based papers have been reported by research groups around the world using the following two common materials deposition approaches:                1. Vacuum sublimation of molecular species; and        2. Dip, spin, and spray coating or printing of oligomeric or polymeric materials.        
Each method has advantages and disadvantages as outlined below.
Vacuum sublimation works well only with relatively low molecular weight (MW) compounds (<600 g/mol). Such compounds must be purified by sublimation or column chromatography to purities >99.99% prior to deposition to obtain superior light emitting efficiencies and device lifetimes. Vacuum sublimation allows for multi-layer configurations and very precise control of film thickness, both of which are advantageous in OLED processing. Drawbacks to vacuum sublimation are that it requires very costly equipment and it is limited to deposition on surface areas that are much smaller than surfaces that can be coated using spin coating. Additionally, device performance is adversely affected by the tendency of some sublimed compounds to crystallize with time. To prevent premature crystallization, compounds are currently being designed with high glass transition temperatures (Tgs) and substituents that minimize or prevent crystallization.
Dip coating, spin coating, spray coating, and printing techniques are generally applicable to the deposition of oligomeric and polymeric materials. It permits precise film thickness control, large area coverage and is relatively inexpensive compared to vacuum sublimation. Multi-layer configurations are only possible if the deposited layers are designed with curable functional groups for subsequent cross-linking, or with differing solubilities to prevent re-dissolution during additional coatings. For example, current OLED polymer technology uses a water soluble prepolymer PPV (shown below), that is thermally cured after deposition rendering it insoluble. [Li, X. C.; Moratti, S. C. Semiconducting Polymers as Light-Emitting Materials; Wise, D. L., Wnek, G. E., Trantolo, D. J., Cooper, T. M. and Gresser, J. D., Ed., 1998]. 
Initial luminescent properties for OLEDs based on polymers were often inferior to their molecular counterparts. This was partly due to the difficulty in obtaining high purity material (99.99%) (polydispersity, endgroups, residual solvents and byproducts such as HCl, catalysts, etc.) necessary for efficient devices. However recent studies have shown that carefully purified polymers have similar or even better resultant properties to their molecular counterparts.
Some important parameters to consider when designing novel materials for practical devices include:    1) Methods of deposition—Spin, spray, dip coating and ink (bubble) jet printing are typically more cost effective than vacuum sublimation. Therefore, the luminescent material is preferably usable with spraying, spin coating, dip coating, and/or printing methods.    2) Materials molecular architecture—materials should be designed to prevent or minimize crystallization and/or aggregation that are known to yield inferior device properties.    3) Color tuning and color purity—Luminescent materials should be designed to provide red, green, and blue (RGB) electroluminescence for full color devices. They should be readily purified to >99.99% purity.    4) Increase in device efficiency, brightness, and lifetime—To provide devices for commercial application, materials should provide >2% external quantum efficiency (2 photons emitted per 100 injected electrons), >500 cd/m2 operating at <5 V, and luminescence half-lives >10,000 hours (roughly equivalent to 10 h/day, 6 days/week for 3 years).    5) Construction of efficient device architectures—The most commonly reported design is shown in FIG. 1. Cathodes are generally prepared by vacuum deposition of Ag/Mg, Ca, or Al. Typically cathodes with lower work functions provide better initial device performance—i.e. Ca<Al/Li/Ag/Mg<Al. However, as Ca, Ag, and Mg are more susceptible to oxidation, Al is generally the cathode material of choice. The anodes are typically commercially available ITO coated glass. Studies show that final device performance is directly correlated to the ITO surface properties; thus, extreme care should be taken when selecting the ITO anode. Alternatively, poly(aniline) (PANI), and poly(2,3-ethylenedioxy)thiophene (PEDOT) have been used as anode material on both ITO deposited on glass and flexible Mylar substrates.
From the foregoing, it will be appreciated that there is a need in the art for OLED materials that can be easily and highly purified, that have high Tgs and little tendency to crystallize or aggregate, that can be processed quickly and efficiently, that are exceptionally resistant to thermal, oxidative, hydrolytic and electrolytic degradation, that can be readily modified to permit tailoring of properties, e.g. stability, electroluminescent efficiencies, solubility, etc., that have low turn-on voltages and relative ease in color tuning.
Nanocomposite Thin Films
To date, the efficient synthesis and processing of multi-layered organic/inorganic nanocomposites remains an elusive goal of the materials chemist. A recent publication reported the rapid, efficient, continuous method to form layered nanocomposites via evaporation induced supramolecular self-assembly (SSA). Sellinger, A., et al., “Continuous self-assembly of organic-inorganic nanocomposite coatings that mimic nacre,” Nature, 1998, 394, 256-60, which is incorporated by reference. During dip coating of homogeneous sols containing alcohol or etheral solvents, silica precursors, organic monomers, initiators, and surfactant (at an initial concentration below the critical micelle concentration [cmc]), solvent evaporation induces the formation of micellar structures that co-organize with silica to form cubic, hexagonal or lamellar mesophases. The organic monomers and initiators are solvated within the hydrophobic micellar interiors. Subsequent photo or thermal polymerization and washing results in a silica/polymer thin film nanocomposite.
This technology can be extended to LED device fabrication by incorporating hole transport, electron transport, and emissive electroluminescent monomers and polymers that will be organized into a layered nanostructure. Preliminary evidence has shown that isolation of electroluminescent materials into layered structures provide an enhanced emissive effect compared to their bulk counterparts. Gin, D. et. al., J. Am. Chem. Soc., 1997, 119(17), 4092-4093.
The exceptional strength, hardness, and toughness of biological nanocomposite systems, composed of seemingly mundane materials, have fueled considerable attention from scientists of many disciplines. Mann, S., Nature, 1993, 365, 499-505. As a result, over 70 such bioceramic nanocomposite materials have been discovered and undoubtedly more will follow. Heuer, A., et al., Science, 1992, 255, 1098-1105.
Of these materials, the most highly studied is that of abalone shell nacre which has an oriented coating composed of alternating layers of aragonite (CaCO3) and biopolymer (˜1 vol %). The organism fabricates the layers with precise microstructure to minimize pores and other defects. As a result, the bioceramic has aesthetic qualities, smooth surface finishes and is two-times harder and 1000-times tougher than their constituent phases. Jackson, A., et al., Proc. R. Soc. Lond. B., 1988, 234, 415-425.
In an attempt to mimic these examples from nature, a synthetic process termed “biomimetics” has gained momentum within the scientific community. Such approaches include crystallization beneath Langmuir monolayers (Fendler, J. et al., Adv. Mater., 1995, 7, 607-632; Heywood, B. et al., Adv. Mater., 1994, 6, 9-19), crystallization on self-assembled monolayers (Tarasevich, B. et al., Chem. Mater., 1996, 8, 292-300), supramolecular self-assembly (SSA) (Yang, H., et al., Mater. Chem., 1997, 7, 1755-1761; Lu, Y., et al., Nature, 1997, 389, 364-368), and sequential deposition (SD) (Keller, S. et al., J. Am. Chem. Soc., 1994, 116, 8817-8818). Of these only SSA and SD offer the ability to introduce the periodic microstructural and compositional changes necessary for layered nanocomposite formation. Lamellar films have been prepared utilizing SSA but these structures collapse upon surfactant removal via washing or thermal processes (Ogawa, M., J. Am. Chem. Soc., 1994, 116, 7941-7942). Stable inorganic/organic nanocomposites (Kleinfeld, E., et al., Science, 1994, 265, 370-373) have been prepared with SD, but this process has some experimental disadvantages, as it requires many repeated deposition steps to build-up a practical coating thickness.
The efficient polymer/silica nanocomposite self-assembly method reported by Sellinger et al., above, is based on a simple spin or dip-coating procedure. The process begins with a homogenous solution of soluble silicates, coupling agent, surfactant, organic monomers, and initiators prepared in either ethanol/water or THF/water solvent systems with an initial surfactant concentration (co) below the critical micelle concentration (cmc). Preferential evaporation of the ethanol or THF during dip-coating progressively enriches the concentrations of water, HCl, and other non-volatile solution components within the depositing film. During solvent evaporation, alcohol soluble organic monomers, and initiators migrate into the hydrophobic portion of the forming micelles. Continued evaporation promotes cooperative assembly of these micellar species into interfacially organized liquid crystalline (LC) mesophases. This organizes both the inorganic and organic precursors simultaneously into the desired structure in a rapid (˜10 sec.) continuous process. Photo- or thermally-induced organic polymerization combined with continued inorganic polymerization locks in the nanocomposite architecture and covalently bonds the organic-inorganic interface.
From the foregoing, it would be an advancement in the art to provide a method of producing a new light emitting device material formed by the self-assembly of organic-inorganic nanocomposite thin films. It would be a further advancement to provide such materials with hole transport, electron transport, and/or emissive compounds, and to fabricate electroluminescent devices from such materials.